Choosing a 4-letter password using only 5 letters that may each be used more than once

Total number of available choices = 625    

Step-by-step explanation:  

Total number of letters available to make a password = 5  

Number of letters to be used in making the password = 4  

Also, it is said that the letters that may each be used more than once.  

⇒ The repetition of letters in the password is allowed

So, choices available for first letter in the password = 5

Choices available for first letter in the password = 5

Choices available for first letter in the password = 5

Choices available for first letter in the password = 5

So, Total number of available choices = 5 × 5 × 5 × 5

                                                                  = 625